Apparatus and method for analyzing blood samples

ABSTRACT

Apparatus for analyzing blood or the like has a centrifuge rotor (10) with a means for visibly holding a sample, and a scanning arm (32) which traverses the rotor and includes a means (38) for sending light to the sample to detect the sample component interfaces. A second light source (40) may be provided for colorimetric inspection of the sample.

APPARATUS AND METHOD FOR ANALYSING BLOOD SAMPLES

This invention relates to improved apparatus and method for analysingblood samples.

For analysis and diagnostic purposes it is known to place a sample ofblood in a haematocrit tube and thereafter spin the tube on a rotor. Thecentrifugal force derived from the spinning action causes the variouscomponents or phases within the blood sample of different density toseparate. Typically the red cells, which are of the highest densitywithin a blood sample, will pack towards one end of the tube. Adjacentto the red cells will be a phase of white blood cells and adjacent tothe white blood cells will be a phase of platelets. Further along thetube there may locate plasma of lighter density.

Once the haematocrit tube has been spun separating the various bloodcomponents it is known to then determine the percentage content or"count" of each component within the sample. To do this effectively theposition of the interface between various phases must be identified withsome level of accuracy.

It has been found however that the interface which occurs between theplatelets and white blood cells can be hard to detect with a sufficientdegree of accuracy. Thus, it is common for operators to estimate theposition of the interface between the platelets and the white bloodcells by experience or expectation rather than analysis oridentification. This estimated percentage is then subtracted from thevolume of both the white blood cell and platelet phases to determine theWhite Blood Cell Count.

The problem with this method is that it does not cater for extraordinarysituations or samples. For example, platelets are known to aggregate andby so doing have a greater density which causes the platelets, when thehaematocrit tube is spun, to mix with the other blood cells. Thus, whenan operator measures the length of the white blood cell and plateletphases, in combination, and thereafter subtracts an estimated plateletvolume there can result a sizeable error if there is no significantplatelet phase because of the aggregation.

This method of determining the white blood cell count is also generallyerroneous or inaccurate when there is an extraordinary high or lowplatelet content in the blood to be analysed.

It is recognised in the present invention that it would be advantageousif a more accurate method of determining the platelet content could bederived, such that the white blood cell count could also be determinedwith greater accuracy.

According to the present invention there is a method for obtaining awhite blood cell count within a blood sample, the method comprising thesteps of:

a) separating the red and white blood cells from the platelets undercentrifugal force, but only to such extent that the platelets aresuspended in a plasma cloud;

b) optically scanning the platelet cloud to measure the extent of lightabsorption and or transmission of the cloud;

c) integrating the measurement obtained from step b) above over thelength of the platelet cloud to determine the content or mass ofplatelets in the blood sample;

d) further spinning the sample to such extent that there is an optimumpacking of the red cells and white blood cells and there is a clearinterface between the red and white blood cells;

e) measuring the volume of the combined packed white blood cell andplatelet phases; and

f) subtracting the platelet content from the content of white bloodcells and platelets in combination to determine the white blood cellcount.

Preferably the blood sample is placed in a haematocrit tube which isfirst spun for approximately 15 seconds at a speed of approximately10,300 revolutions per minute. This separates the cellular and aqueouscomponents of the blood sample.

Preferably also the optical density of the various phases is determinedby measuring the percentage transmission of an optic beam originatingfrom an optical source, such as an infra-red laser, through the sample.Advantageously, this is performed while the centrifuge rotor rotates ata reduced speed of approximately 1000 revolutions per minute, duringwhich a scanning arm holding the optical light source moves across thecentrifuge rotor.

Preferably after the content of platelets has been determined the sampleis again spun at a circular velocity of approximately 10300 revolutionsper minute and for a duration of approximately five minutes to achieveoptimum packing and separation of the red and white blood cell phases.

Apparatus which may be used in performing the invention is shown in theaccompanying figures, wherein:

FIG. 1 illustrates a centrifuge and scanning arm; and

FIGS. 2a and 2b show an haematocrit tube containing a sample of bloodwherein the components thereof have been separated to differing extentunder centrifugal force.

The Figures also include FIG. 3 which shows a typical analogue tracealong the length of a haematocrit tube showing the detection and natureof interfaces by the extent of optical transmission through each phase.

With reference to the Figures, an haematocrit tube 1 as shown in FIG. 2may be partially filled with a blood sample and then sealed, such as byplugging with a putty, or by heat sealing. The haematocrit tube 1 maythen be placed in a groove 2 of the centrifuge rotor 3, illustrated inFIG. 1. The centrifuge is geared and empowered to rotate at a range ofspeeds up to and in excess of 10,000 revolutions per minute.

Associated with the centrifuge rotor 3 is a scanning arm 4 embodyingvarious optical sources. In particular the scanning arm 4 houses aninfra-red laser 5 and a polychromatic visible light source (not shown).

In accordance with the present invention the tube 1 located in thegroove 2 is spun at a high speed, such as 10,000 revolutions per minute,but only for a relatively short period of time of say 10 to 20 seconds.Although this does not achieve full packing of the cells it doesdesirably provide separation of the cellular and aqueous components ofthe sample, desirably providing the sample to take a form similar tothat shown in FIG. 2a. It may be seen in this Figure that the whiteblood cells 6 and the red blood cells 7 are held at one end of the tube1, while the platelets are suspended in a plasma cloud 8 toward theother end of the tube 1.

Desirably the centrifuge is then slowed to a speed appropriate forscanning the sample, and if apparatus as shown in FIG. 1 is employed,the scanning arm 4 would then pass over the tube 1 and, in particular,the infrared laser shone through the cloud, enabling the measurement ofthe transmission of the optical energy through the cloud 8, giving anindication of the cloud's density.

An algorithm relating the density of the cloud 8 to the extent ofoptical transmission can be used in conjunction with the dimensions ofthe cloud 8 to determine the percentage of platelets in the sample.

The tube is then spun at the higher velocity again to promote fullseparation of each cell type and sufficient packing to allow foraccurate Packed Cell Volume measurements to be taken. The sample mightthen be in the form illustrated in FIG. 2b. This could be achieved byspinning the tube 1 again at a higher speed of approximately 10 to10,500 revolutions per minute and for approximately 5 minutes. In FIG.2b the blood sample comprises the following separated components; packedred cells 7, white blood cells 6, platelets 9, plasma 10 and air 11.

FIG. 3 shows an analogue trace of the sample in the haematocrit tube 1shown in FIG. 2b. From an analogue trace it is generally possible todetect the position and nature of the interfaces between the components.However, it is not always clear where the interface is between theplatelets 9 and the white blood cells 6. As the invention allows for therelatively accurate determination of the percentage content of plateletsin the sample, the dimensions of the white blood cells 6 and platelets 9can be measured in combination and thereafter the platelet content besubtracted, giving a more reliable white blood cell count than thatheretofore possible.

Modifications in the method are possible without departing from theintended scope of the invention, particularly in respect of thetechniques, speeds or duration of spinning the sample to achieve thecomponent separation.

What is claimed is:
 1. A method for obtaining a white blood cell countwithin a blood sample, the method comprising the steps of:a) separatingthe red and white blood cells from the platelets under centrifugalforce, but only to such extent that the platelets are suspended in aplasma cloud; b) optically scanning the platelet cloud to measure theextent of light absorption and/or transmission of the cloud; c)integrating the measurement obtained from step b) above over the lengthof the platelet cloud to determine the content or mass of platelets inthe blood sample; d) further spinning the sample to such extent thatthere is an optimum packing of the red cells and white blood cells andthere is a clear interface between the red and white blood cells; e)measuring the volume of the combined packed white blood cell andplatelet phases; and f) subtracting the platelet content from thecontent of white blood cells and platelets in combination to determinethe white blood cell count.
 2. A method as claimed in claim 1 whereinthe blood sample is placed in a haematocrit tube which is first spun forapproximately 15 seconds at a speed of approximately 10,300 revolutionsper minute.
 3. A method as claimed in claim 1 or claim 2 wherein theoptical density of the platelet cloud is determined by measuring thepercentage transmission of an optic beam originating from an opticalsource through the sample.
 4. A method as claimed in claim 3 wherein theoptic beam is passed through the sample while the centrifuge rotates ata reduced speed of approximately 1,000 revolutions per minute.
 5. Amethod as claimed claim 1 wherein after the content of platelets hasbeen determined, the sample is again spun at a circular velocity ofapproximately 10,300 revolutions per minute and for a duration ofapproximately 5 minutes.